水量热法加速器光子水吸收剂量绝对测量与国际比对

为实现加速器光子水吸收剂量绝对测量,研制了水量热计系统,在此基础上取得国际互认并建立加速器光子水吸收剂量基准,进一步提高了我国放疗剂量量值传递能力。通过水浴与半导体制冷系统二级控温,将量热计水模体的温度漂移控制在0.5μ℃/s。利用惠斯通交流电桥测量辐射所致的热敏探针阻值变化,逐次校准热敏探针和交流电桥,实现了医用加速器光子水吸收剂量的绝对测量,合成标准不确定度为0.30%。参加了国际计量局加速器光子水吸收剂量关键比对,复现的6MV和10MV光子水吸收剂量值与比对参考值之比分别为0.9917和0.9949,在不确定度允许的范围内一致。

Absolute Measurement and International Comparison of Absorbed Dose to Water for Accelerator Photon Beams by Water Calorimeter

A water calorimeter was developed for conducting the absolute measurement of the absorbed dose to water of the photon beams from accelerator, followed by obtaining the international mutual recognition and establishing the primary standard for absorbed dose to water of the photo beams from accelerator and improving the dissemination for radiotherapy in China. The temperature drift of water phantom of the calorimeter was maintained at 0.5μ℃/s by the bath and Peltier cooling system. The resistance of thermistors caused by radiation was measured by Wheatstone AC bridge. After calibrating thermistors and the bridge separately,the absorbed dose to water of photon beams from accelerator was reproduced with the uncertainty of 0.30%. The key comparison on the absorbed dose to water of the photon beams from accelerator was performed between NIM and BIPM. By considering the result of NIM as against the reference quantity of BIPM, the ratio for the 6MV and 10MV photon beams is 0.9917 and 0.9949 respectively, which is within the acceptance of uncertainty.